By the end of August, the nuclear-powered rover Curiosity will take its first, tentative steps on the surface of Mars, rolling a meter or two forward on its six aluminum wheels across the flat floor of Gale Crater. By then, controllers at the Jet Propulsion Laboratory here probably will have finished checking out Curiosity's complex systems and calibrating its suite of 10 sophisticated instruments. With plans to spend at least two years exploring the crater and the 18,000-ft. mountain in its center, they are in no hurry ...